Groups of Years

How Often?

Annual: Yearly

Biannual: Twice a year (not equally spaced)

Semiannual: Twice a year equally spaced; every 6 months

Semicentennial: Every 50 years

Centennial: Every 100 years

The Aztec and Mayan Calendars

The people of these ancient civilizations used two calendars. One was a sacred calendar of 260 days that marked religious feasts. The other was a secular (not religious) calendar of 365 days divided into 18 months of 20 days each. The extra 5 days were added throughout the years, much like the time we add each leap year. Today, many people follow a religious calendar as well as the secular one.

Religious Calendars

The Jewish New Year, called Rosh Hashanah, occurs on the first and second day of the Hebrew month Tishri, which can come in either September or October.

The Islamic calendar is based on a lunar (moon) year of 354 days. It is calculated from the Hegira, in A.D. 622, and grouped in 30-year cycles. (The Hegira was the flight from Mecca by Muhammed, the founder of Islam, to escape persecution.)